1. Technical Field
The invention relates to an electrophotographic photoreceptor which is used in a copier, and the like, which forms an image by an electrophotographic method, as well as a process cartridge and an image forming device using the same.
2. Related Art
Recently, electrophotography is widely used for a copier, a printer, and the like. An electrophotographic photoreceptor (hereunder, also called a “photoreceptor”) used for an image forming device utilizing such electrophotography is subject to various contact or stress in the device, causing deterioration. However, on the other hand, high reliability is desired together with digitalization and colorization of the image forming device.
For example, if a charging process of a photoreceptor is addressed, there are problems as follows. Firstly, in a noncontact charging method, discharged products are adhered to the photoreceptor, and image blur and the like occurs. Therefore, in order to remove the discharged products adhered to the photoreceptor, there is employed for example a system where particles having an abrasion function are mixed in developer and the discharged products are rubbed off in a cleaning section. In this case, the surface of the photoreceptor is deteriorated due to abrasion. On the other hand, recently, a contact charging method is widely used. In this method, abrasion of the photoreceptor may be also accelerated.
From such a background, a longer lifetime is desired for an electrophotographic photoreceptor. Since abrasion resistance is needed to be improved for a longer lifetime of the electrophotographic photoreceptor, the hardness of the photoreceptor surface is required to be increased.
However, in a photoreceptor comprising a hard amorphous silicon surface, discharged products are adhered, and image blur and image flow are easily generated. This phenomenon is remarkable particularly in a high-humidity environment. It is similar regarding the surface layer of an organic photoreceptor having an organic photosensitive layer.
In order to suppress the generation of such problems, a material of a carbon system is often used as the surface layer of the photoreceptor.
However, in a carbon based film such as a hydrogenated amorphous carbon (a-C:H), and a fluoridated film thereof (a-C:H, F), there is a tendency that the film is colored as the film hardness is improved. Consequently, if the surface layer comprising a carbon based film is worn by use, the transmitted light volume through the surface layer is increased with time, causing a problem of increasing the sensitivity of the photosensitive layer provided inside of the surface layer. Moreover, if the surface layer is unevenly worn in the surface direction, the sensitivity of the photosensitive layer becomes uneven. Thus there is a problem of readily causing image blur especially when forming a half tone image.
On the other hand, as a general characteristic of a carbon based thin film material, it is known that hardness improvement and transparency improvement is in a trade off relation. The reason is that, when the carbon bond in the film is addressed, it is necessary to increase the diamond type sp3 bondability in order to improve the hardness. However it is inevitable that graphite type sp2 bonds absorbing light are mixed in the film, and furthermore when an attempt is made to reduce the presence of the graphite type sp2 bonds by adding hydrogen into the film or the like, the transparency is improved but the film property becomes organic and the hardness is decreased.
Moreover, recently research and development of a carbon nitride film has been conducted. However it has not yet reached to a hardness or a characteristic more than a conventionally known carbon based thin film such as a diamond film and a diamond like carbon film. In order to obtain a harder and more minute film, it is necessary to heat at about 1000° C. and increase the discharge power at the time of forming the film. However, such a film forming method requiring a discharge condition of high temperature and high energy, is difficult to apply to an organic photoreceptor susceptible to damage by heat or discharge, and is thus not practical.
In this manner, from the point of compatibility of the hardness and the transparency, a conventional carbon based thin film is insufficient as a surface layer of a photoreceptor. On the other hand, from this point, a hydrogenated amorphous silicon carbide film (a-SiC:H) is superior. However, since image blur and image flow easily occur due to adhesion of discharged products, it is necessary to use a drum heater in order to suppress the occurrence thereof.
Furthermore, a hydrogenated nitride semiconductor is superior in hardness and transparency but lacks moisture resistance, and is inferior in practicality under a high humidity environment.
On the other hand, in contrast to a method of forming a surface layer utilizing film-formation in the vapor phase as mentioned above, there is proposed methods of forming a surface layer by means of coating. Among these, in order to improve the abrasion resistance, it is known to use a high molecule compound having a siloxane bond for the surface layer. However, a surface layer comprising such a material is less hard compared to a surface layer formed utilizing film-formation in the vapor phase. As a result, when the photoreceptor surface is blemished or abraded with time, the surface adhesiveness is increased and the toner is adhered onto the photoreceptor surface, causing a problem of shortening the photoreceptor lifetime.